"Lullaby" is a love poem spoken to someone who's sleeping. Romantic or Creepy? You be the judge. The speaker begins by telling his lover to lay his head on the speaker's arm. He then takes a minute to think about the fact that beauty can't last forever because death comes for us all in the end. Hmmm, depressing. He concludes that even his lover isn't perfect, but it doesn't matter: he's "entirely beautiful" anyway.
Then the speaker gets a little pensive; he starts to think about a bunch of random things, like Venus (the Roman goddess of love), a hermit who has an ecstatic experience, and madmen who bemoan the future. But despite all these (usually negative) wandering thoughts, he doesn't want to forget anything that has happened on this beautiful night with his lover.
The speaker says one more time that everything dies eventually (we get the point, Mr. Depressing), but then he begins to speak directly to his sleeping beloved. He prays (in the least religious way possible) that his beloved will experience life and "find the mortal world enough." Basically, he hopes his lover won't go looking too far for the answers to life's big questions. At the end of the poem, he says that the man is "watched by every human love." It's not God who looks after his sleeping beloved; it's the speaker himself.